1. Field of the Invention
The present invention relates to an adsorbent deterioration determining apparatus for determining a deterioration of an adsorbent which is arranged in an exhaust system of an internal combustion engine for adsorbing hydrocarbons in exhaust gases to purify the exhaust gases.
2. Description of the Prior Art
A conventional adsorbent deterioration determining apparatus of the type mentioned above is known and disclosed, for example, in Laid-open Japanese Patent Application No. 11-2115. This conventional determining apparatus is provided with temperature sensors arranged at locations upstream and downstream of an adsorbent, respectively, for determining a deterioration of the adsorbent by comparing times at which both the temperature sensors generate peaks of detected values, or comparing times at which both the temperature sensors generate peaks of detected values differentiated by time, while the adsorbent is in adsorbing conditions. Such a determining approach relies on the nature of the adsorbent for determining the deterioration of the adsorbent. Specifically, in the adsorbing conditions, the adsorbent presents a peak of the internal temperature or its rising rate, and the peak occurs later at a downstream location than at an upstream location, and the peak occurs earlier at the downstream location as the adsorbent is deteriorated. In this manner, the adsorbent can be determined for deterioration based on a comparison of times at which both the temperature sensors detect the peak values at their respective locations, rather than a comparison of temperature values themselves detected by the temperature sensors, without using highly accurate temperature sensor.
However, the conventional determining apparatus mentioned above merely evaluates the state of the adsorbent indirectly based on a change in the internal temperature of the adsorbent associated with the adsorbing action of the adsorbent, using as determination parameters the times at which the peaks of the internal temperatures or its rising rate are detected by the temperature sensors. Since this conventional determining apparatus does not directly sense an adsorbing state of the adsorbent, the determining apparatus is susceptible to errors and is incapable of accurately determining a deterioration of the adsorbent. In addition, due to the requirement of two temperature sensors, which are relatively expensive, the determining apparatus suffers from a corresponding increase in the manufacturing cost.
The present invention has been made to solve the problems as mentioned above, and it is an object of the invention to provide an adsorbent deterioration determining apparatus which is capable of accurately determining a deterioration of an adsorbent in a relatively simple configuration.
To achieve the above object, the present invention provides an adsorbent deterioration determining apparatus for determining a deterioration of an adsorbent arranged in an exhaust system of an internal combustion engine for purifying exhaust gas, where the adsorbent capable of adsorbing hydrocarbons and moisture in exhaust gases. The adsorbent deterioration determining apparatus is characterized by comprising a humidity sensor arranged at a location downstream of the adsorbent in the exhaust system for detecting a humidity of exhaust gases; and deterioration determining means for determining a deterioration of the adsorbent in accordance with a detected output of the humidity sensor at the time a predetermined time has elapsed from a start of the internal combustion engine.
According to this adsorbent deterioration determining apparatus, the humidity sensor is arranged at a location downstream of the adsorbent in the exhaust system for detecting the humidity of exhaust gases, such that a deterioration of the adsorbent is determined in accordance with a detected output of the humidity sensor at the time the predetermined time has elapsed from the start of the internal combustion engine. The adsorbent is capable of adsorbing moisture as well as hydrocarbons, and its abilities to adsorb hydrocarbons and moisture are in a close relationship with each other, so that the humidity of exhaust gases at a location downstream of the adsorbent, detected by the humidity sensor, well reflects the condition in which the adsorbent adsorbs hydrocarbons.
Also, the humidity of exhaust gases downstream of the adsorbent does not rise in an initial stage after the internal combustion engine is started because moisture included in exhaust gases is adsorbed by the adsorbent together with hydrocarbons. Subsequently, the humidity rises as the adsorbent is gradually saturated, allowing the moisture in exhaust gases to incrementally pass through the adsorbent. When the adsorbent is deteriorated, a degradation in its adsorbing abilities allows moisture in exhaust gases to pass through the adsorbent earlier, so that the humidity of exhaust gases downstream of the adsorbent rises at a timing earlier than when the adsorbent is not deteriorated. Therefore, a deterioration of the adsorbent can be determined in accordance with the detected output of the humidity sensor at the time the predetermined time has elapsed from the start of the internal combustion engine to provide an accurate deterioration determination, while well reflecting the condition in which the adsorbent adsorbs hydrocarbons. In addition, since only one humidity sensor, which is generally less expensive than a temperature sensor, need be arranged at a location downstream of the adsorbent, the resulting determining apparatus can be made in a relatively simple configuration at a low cost.
Preferably, in the foregoing adsorbent deterioration determining apparatus, the humidity sensor includes a sensor element which has an impedance characteristic that varies in accordance with the humidity, and outputs as the detected output a voltage in accordance with a change in the impedance characteristic when the sensor element is applied with a supply voltage.
According to this preferred embodiment of the adsorbent deterioration determining apparatus, the sensor element forming part of the humidity sensor presents the impedance characteristic that varies in accordance with the humidity. A change in the impedance characteristic is read as a voltage by applying the sensor element with the supply voltage, and outputted as the detected output of the humidity sensor. Thus, this output voltage can be used in the deterioration determination as a parameter indicative of the humidity of exhaust gases downstream of the adsorbent. The output voltage generated in this manner is sufficient in regard to the accuracy for detecting a change in the humidity of exhaust gases required for determining the deterioration of the adsorbent. Also, since the sensor element is only required to include an additional component for applying a voltage thereto, the humidity sensor can be implemented in a simple circuit configuration at a low cost.
Preferably, in the adsorbent deterioration determining apparatus, the humidity sensor includes a sensor element which has an impedance characteristic that varies in accordance with the humidity, and an oscillator for generating an oscillating wave at a frequency defined in accordance with the impedance characteristic of the sensor element, wherein the humidity sensor outputs as the detected output the oscillating wave from the oscillator.
According to this preferred embodiment of the adsorbent deterioration determining apparatus, the sensor element forming part of the humidity sensor presents the impedance characteristic that varies in accordance with the humidity, and the oscillator outputs the oscillating wave having a frequency characteristic in accordance with the impedance characteristic as the detected output of the humidity sensor. Thus, the frequency characteristic of the oscillating wave can be used in the deterioration determination as a parameter indicative of the humidity of exhaust gases downstream of the adsorbent. In addition, a period, for example, can be used as the frequency characteristic, in which case a deterioration can be more accurately determined for the adsorbent because of the detection accuracy which can be more readily ensured, as compared with an embodiment which involves an A/D conversion of the output voltage value or the like.
Preferably, the adsorbent deterioration determining apparatus further comprises temperature detecting means for detecting a temperature around the humidity sensor; and relative humidity calculating means for calculating a relative humidity of exhaust gases based on the detected output of the humidity sensor, and the temperature detected by the temperature detecting means, wherein the deterioration determining means determines a deterioration of the adsorbent in accordance with the calculated relative humidity.
According to this preferred embodiment of the adsorbent deterioration determining apparatus, the relative humidity compensated for the temperature can be appropriately calculated based on the detected output of the humidity sensor and the temperature detected by the temperature detecting means. In addition, a deterioration of the adsorbent can be accurately determined in accordance with the relative humidity thus calculated.